Linear low density polyethylene (LLDPE) is a copolymer of ethylene with a minor amount of comonomers, typically acyclic C3-C8 α-olefins. The short-chain branching in such polymers in an otherwise linear backbone gives LLDPE unique mechanical properties and processing attributes compared with highly branched low density polyethylene (LDPE) produced by a high pressure radical process, and unbranched high density polyethylene (HDPE) made from a low pressure metal-catalyzed process. Due to the relatively low cost and its satisfactory overall mechanical properties, LLDPE is widely employed as the major component in films. However, the lack of some rheological characteristics such as shear thinning, strain hardening, and melt strength gives rise to processing difficulty in fabrication of LLDPE into films by techniques such as blown bubble extrusion, or in making foamed articles. Typically, LDPE is added to LLDPE to improve its melt strength and bubble stability, but at the same time some mechanical properties such as impact toughness are compromised.
Various approaches around the combination and optimization of long-chain branching structure and compositional/molecular weight distributions have been explored, but the incremental gains obtained are difficult to implement commercially. A norbornene comonomer approach was previously explored in U.S. Pat. No. 5,942,587 to make poly(ethylene-co-α-olefin-co-cyclic olefin) terpolymers, or “cyclic olefin copolymers” (COC's). The COC's produced by a gas-phase process and heterogeneous catalysis had significantly improved tensile strength and modulus and Elmendorf tear properties, but decreased Dart Drop impact. The gas-phase COC's had relatively broad composition and molecular weight distributions but it is not clear from this reference if the COC's are linear or branched. The gas-phase COC's also showed minimal to modest improvements in shear thinning and melt strength. Thus, the gas-phase COC's didn't exhibit an ideal balance of mechanical properties. What is needed is a polyethylene having a better balance of properties when made into useful films and other articles, while having improved processability.
Other references include U.S. Pat. Nos. 5,087,677; 5,635,573; 5,629,398; 9,321,911; and US 2003/0130452.